The present invention relates generally to the fabrication of single crystal silicon substrates, and more particularly, to a process for fabricating single crystal silicon wafers having oxygen precipitate nucleation centers which can be stabilized and serve as a site for the growth of oxygen precipitates with the number of oxygen precipitates having a low order of dependance upon the oxygen concentration of the single crystal silicon.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared with the so-called Czochralski ("Cz") process wherein a single seed crystal is immersed into molten silicon and then grown by slow extraction. At the temperature of the silicon molten mass, oxygen comes into the crystal lattice from the quartz crucible in which it is held until it reaches a concentration determined by the solubility of oxygen in silicon at the temperature of the molten mass and by the actual segregation coefficient of oxygen in solidified silicon. Such concentrations are greater than the solubility of oxygen in solid silicon at the temperatures typical for the processes for the fabrication of integrated circuits. As the crystal grows from the molten mass and cools, therefore, the solubility of oxygen in it decreases rapidly, whereby in the resulting slices or wafers, oxygen is present in supersaturated concentrations.
Thermal treatment cycles which are typically employed in the fabrication of electronic devices can cause the precipitation of oxygen in silicon wafers which are supersaturated in oxygen. Depending upon their location in the wafer, the precipitates can be harmful or beneficial. Oxygen precipitates located in the active device region of the wafer can impair the operation of the device. Oxygen precipitates located in the bulk of the wafer, however, are capable of trapping undesired metal impurities that may come into contact with the wafer. The use of oxygen precipitates located in the bulk of the wafer to trap metals is commonly referred to internal or intrinsic gettering ("IG").
Because of the problems associated with oxygen precipitates in the active device region, electronic device fabricators must use silicon wafers which are incapable of forming oxygen precipitates anywhere in the wafer under their process conditions, or alternatively, wafers which only form oxygen precipitates in the bulk of the wafer under their process conditions. Many electronic device fabricators prefer the latter alternative in view of the benefits of IG.
In general, the electronic device fabrication process inherently includes a series of steps which, in principle, can be used to form a zone near the surface of the wafer which is free of oxygen precipitates (commonly referred to as a "denuded zone" or a "precipitate free zone") with the balance of the wafer containing a sufficient number of oxygen precipitates for IG purposes. Denuded zones can be formed, for example, in a high-low-high thermal sequence such as (a) oxygen outdiffusion heat treatment at a high temperature (&gt;1100.degree. C.) in an inert ambient for a period of at least about 4 hours, (b) oxygen precipitate nuclei formation at a low temperature (600.degree.-750.degree. C.), and (c) growth of oxygen (SiO.sub.2) precipitates at a high temperature (1000.degree.-1150.degree. C.). See, e.g., F. Shimura, Semiconductor Silicon Crystal Technology, Academic Press, Inc., San Diego Calif. (1989) at pages 361-367 and the references cited therein.
A critical requirement for many electronic device fabricators is that all wafers subjected to their thermal sequence have a uniform and reproducible denuded zone and a uniform and reproducible number density of oxygen precipitates outside the denuded zone. Uniformity and reproducibility have been difficult to achieve at a reasonable cost, however. There are several parameters which affect the density of oxide precipitates which develop in a given silicon wafer in a given IC manufacturing process, including: (1) the concentration of interstitial oxygen, [O.sub.i ].sub.i present initially in solid solution, (2) the density of pre-existing (to the IC manufacturing process) oxygen clusters which act as nucleation sites for the precipitation of supersaturated oxygen, (3) the stability of these pre-existing clusters at higher temperatures, and (4) the details of the thermal cycles employed to produce the electronic device. These parameters can vary significantly from one wafer to the next.
One approach which has been tried to control the range in the concentration of oxygen precipitates formed during an IC manufacturing process is to narrow the range of oxygen concentration for the wafers. For example, many IC fabricators require that the range on oxygen concentration be within 1 ppma of a target value, or even less. This approach, however, stretches technological capability, reduces the flexibility of crystal growers to control other parameters and increases costs. Even worse, tightening oxygen concentration specifications does not guarantee success; thermal histories of the silicon wafers can have a profound effect upon the oxygen precipitation behavior. Thus, wafers having the same oxygen concentrations but different thermal histories can exhibit significantly different precipitate densities.
In view of the fact that tightening oxygen concentration specifications by itself will not lead to a narrow range of oxygen precipitate densities, some have attempted sorting wafers by oxygen concentration or other criteria from which values of oxygen precipitation values can be predicted. See, for example, Miller U.S. Pat. No. 4,809,196. Wafer-to-wafer uniformity with respect to oxygen precipitation is improved by this approach, but flexibility is significantly impaired and costs are increased.
Bischoff et al. suggest a process for forming wafers having a wide denuded zone (.gtoreq.15 .mu.m) with a high precipitate density (&gt;10.sup.12 /cm.sup.3) in U.S. Pat. No. 4,437,922. In their process, the denuded zone is formed first by annealing the wafers at 1100.degree. C. for four hours. After the denuded zone is formed, Bischoff et al. suggest that the wafers be annealed at temperature in the range of 400.degree. to 500.degree. C. to nucleate a high density of very small precipitates and grow them to such a size to permit survival of a subsequent heat treatment such as 925.degree. C. Thereafter, Bischoff et al. suggest heating the wafers at a rate of less than 2.degree. C. per minute to a temperature between 750.degree. C. and 1000.degree. C. and annealing the wafers at this temperature for a period which is sufficient to ensure the survival of the precipitates in subsequent processing.
A significant disadvantage of the Bischoff et al. process is its failure to take advantage of the high temperature steps which are routinely used in electronic device fabrication. These steps could be used to form the denuded zone and to stabilize the oxygen precipitates in the balance of the wafer, provided the precipitation behavior were tailored for the specific electronic device fabrication process. The additional steps required by Bischoff et al. add significant labor and cost to the wafers. Furthermore, nowhere do Bischoff et al. disclose any means for controlling the number density of the oxygen precipitates; Bischoff et al. merely disclosed how to consistently obtain a high density (&gt;10.sup.12 /cm.sup.3) which may not be appropriate for many applications.